Lithographic apparatus and device manufacturing method for measuring wafer parameters using non-standard alignment settings

ABSTRACT

A method produces at least one monitor wafer for a lithographic apparatus. The monitor wafer is for use in combination with a scanning control module to periodically retrieve measurements defining a baseline from the monitor wafer thereby determining parameter drift from the baseline. In doing this, allowance and/or correction can be to be made for the drift. The baseline is determined by initially exposing the monitor wafer(s) using the lithographic apparatus, such that the initial exposure is performed while using non-standard alignment model settings optimized for accuracy, such as those used for testing the apparatus. An associated lithographic apparatus is also disclosed.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims benefit under 35 U.S.C. 119(e) to U.S.Provisional Patent Application No. 61/306,029, filed Feb. 19, 2010,which is incorporated by reference herein in its entirety.

FIELD

Embodiments of the present invention relate to methods and apparatususable, for example, in the manufacture of devices by lithographictechniques.

BACKGROUND

A lithographic apparatus is a machine that applies a desired patternonto a substrate, usually onto a target portion of the substrate. Alithographic apparatus can be used, for example, in the manufacture ofintegrated circuits (ICs). In that instance, a patterning device, whichis alternatively referred to as a “mask” or a “reticle,” may be used togenerate a circuit pattern to be formed on an individual layer of theIC. This pattern can be transferred onto a target portion (e.g.,including part of, one, or several dies) on a substrate (e.g., a siliconwafer). Transfer of the pattern is typically via imaging onto a layer ofradiation-sensitive material (resist) provided on the substrate. Ingeneral, a single substrate will contain a network of adjacent targetportions that are successively patterned. Known lithographic apparatusinclude so-called steppers, in which each target portion is irradiatedby exposing an entire pattern onto the target portion at one time, andso-called “scanners,” in which each target portion is irradiated byscanning the pattern through a radiation beam in a given direction (the“scanning”-direction) while synchronously scanning the substrateparallel or anti-parallel to this direction. It is also possible totransfer the pattern from the patterning device to the substrate byimprinting the pattern onto the substrate.

In order to monitor the lithographic process, parameters of thepatterned substrate are measured. Parameters may include, for example,the overlay error between successive layers formed in or on thepatterned substrate and critical linewidths of developed photosensitiveresist. This measurement may be performed on a product substrate and/oron a dedicated metrology target. There are various techniques for makingmeasurements of the microscopic structures formed in lithographicprocesses, including the use of scanning electron microscopes andvarious specialized tools. A fast and non-invasive form of specializedinspection tool is a scatterometer in which a beam of radiation isdirected onto a target on the surface of the substrate and properties ofthe scattered or reflected beam are measured. By comparing theproperties of the beam before and after it has been reflected orscattered by the substrate, the properties of the substrate can bedetermined. This can be done, for example, by comparing the reflectedbeam with data stored in a library of known measurements associated withknown substrate properties. Two main types of scatterometers are known.Spectroscopic scatterometers direct a broadband radiation beam onto thesubstrate and measure the spectrum (intensity as a function ofwavelength) of the radiation scattered into a particular narrow angularrange. Angularly-resolved scatterometers use a monochromatic radiationbeam and measure the intensity of the scattered radiation as a functionof angle.

In order to better control scanner functionality, a module has beenrecently devised which automatically drives the system towards apre-defined baseline approximately each day. This scanner stabilitymodule retrieves standard measurements taken from a monitor wafer usinga metrology tool. The monitor wafer had been previously exposed using aspecial reticle containing special scatterometry marks. Using themonitor wafer and that day's measurements (and possibly historicalmeasurement data from previous days), the scanner stability moduledetermines how far the system has drifted from its baseline, and thencalculates wafer-level overlay and focus correction sets. The baselinecan be defined either directly by the reference layer on the monitorwafer (in this case, the scanner stability module will drive the systemtowards minimal overlay on the baseline monitor wafers) or indirectly bya combination of the reference layer on the wafers and a target overlayfingerprint (in this case, the scanner stability module will drive thesystem towards the defined target overlay fingerprint on the monitorwafers). The lithography system then converts these correction sets intospecific corrections for each exposure on subsequent production wafers.

The alignment model sequences produce a significant noise source for thescanner stability module controller, which attempts to control thescanner using overlay data from the very limited number of monitorwafers (typically from 4 to 12 wafers per week for each scanner).

SUMMARY

It is desirable to provide a system whereby the scanner stability modulecontrol accuracy is improved while still using the same or similarlimited number of monitor wafers as present.

According to an aspect of an embodiment of the present invention, thereis provided a lithographic apparatus that includes the following: anillumination system configured to condition a radiation beam, a supportconstructed to support a patterning device, the patterning device beingcapable of imparting the radiation beam with a pattern in itscross-section to form a patterned radiation beam, a substrate tableconstructed to hold a substrate, a projection system configured toproject the patterned radiation beam onto a target portion of thesubstrate, and, a scanning control module operable to aid control of atleast one of the support, substrate table or projection system byperiodically retrieving measurements defining baseline controlparameters from at least one reference substrate so as to determineparameter drift from the baseline control parameters thereby enablingallowance and/or correction to be made for the drift, the referencesubstrate having been initially exposed so as to determine the baselinecontrol parameters, wherein the apparatus is operable in at least ascanning mode wherein the patterned radiation beam is scanned across thetarget portion of the substrate, the apparatus being further operable,during the initial exposing of the at least one reference substrate, toselect non-standard alignment model settings optimized for accuracy.

According to a second aspect of an embodiment of the present invention,there is provided a method of producing at least one reference substratefor a lithographic apparatus, the reference substrate being usable incombination with a scanning control module which periodically retrievesmeasurements from the least one reference substrate which definebaseline control parameters, so as to aid scanning control during alithographic process by determining parameter drift from the baselinecontrol parameters, thereby enabling allowance and/or correction to bemade for the drift, the baseline control parameters being determined byinitially exposing the at least one reference substrate using thelithographic apparatus, the method comprising performing the initialexposure while using non-standard alignment model settings optimized foraccuracy.

Further features and advantages of embodiment of the present invention,as well as the structure and operation of various embodiments of theinvention, are described in detail below with reference to theaccompanying drawings. It is noted that the invention is not limited tothe specific embodiments described herein. Such embodiments arepresented herein for illustrative purposes only. Additional embodimentswill be apparent to persons skilled in the relevant art(s) based on theteachings contained herein.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated herein and form partof the specification, illustrate embodiments of the present inventionand, together with the description, further serve to explain theprinciples of embodiments of the invention and to enable a personskilled in the relevant art(s) to make and use embodiments of thepresent invention.

FIG. 1 depicts an example lithographic apparatus, which may be used withembodiments of the present invention.

FIG. 2 depicts an example lithographic cell or cluster, which may beused with embodiments of the present invention.

FIG. 3 depicts a first example of a scatterometer, which may be usedwith embodiments of the present invention.

FIG. 4 depicts a second example of a scatterometer, which may be usedwith embodiments of the present invention.

FIG. 5 illustrates an embodiment of control loops in a lithographicprocess utilizing a scanner stability module.

The features and advantages of embodiments of the present invention willbecome more apparent from the detailed description set forth below whentaken in conjunction with the drawings, in which like referencecharacters identify corresponding elements throughout. In the drawings,like reference numbers generally indicate identical, functionallysimilar, and/or structurally similar elements. The drawing in which anelement first appears is indicated by the leftmost digit(s) in thecorresponding reference number.

DETAILED DESCRIPTION

This specification discloses one or more embodiments that incorporatethe features of this invention. The disclosed embodiment(s) merelyexemplify the invention. The scope of the invention is not limited tothe disclosed embodiment(s). The invention is defined by the claimsappended hereto.

The embodiment(s) described, and references in the specification to “oneembodiment”, “an embodiment”, “an example embodiment”, etc., indicatethat the embodiment(s) described may include a particular feature,structure, or characteristic, but every embodiment may not necessarilyinclude the particular feature, structure, or characteristic. Moreover,such phrases are not necessarily referring to the same embodiment.Further, when a particular feature, structure, or characteristic isdescribed in connection with an embodiment, it is understood that it iswithin the knowledge of one skilled in the relevant art to effect suchfeature, structure, or characteristic in connection with otherembodiments whether or not explicitly described.

Embodiments of the invention may be implemented in hardware, firmware,software, or any combination thereof. Embodiments of the invention mayalso be implemented as instructions stored on a machine-readable medium,which may be read and executed by one or more processors. Amachine-readable medium may include any mechanism for storing ortransmitting information in a form readable by a machine (e.g., acomputing device). For example, a machine-readable medium may includethe following: read only memory (ROM); random access memory (RAM);magnetic disk storage media; optical storage media; flash memorydevices; and, electrical, optical, acoustical signals, and others.Further, firmware, software, routines, instructions may be describedherein as performing certain actions. However, it should be appreciatedthat such descriptions are merely for convenience and that such actionsin fact result from computing devices, processors, controllers, or otherdevices executing the firmware, software, routines, instructions, etc.

Before describing such embodiments in more detail, however, it isinstructive to present an example environment in which embodiments ofthe present invention may be implemented.

FIG. 1 schematically depicts an example lithographic apparatus, whichmay be used with embodiments of the present invention. The apparatusincludes the following an illumination system (illuminator) ILconfigured to condition a radiation beam B (e.g., UV radiation or DUVradiation), a support structure (e.g., a mask table) MT constructed tosupport a patterning device (e.g., a mask) MA and connected to a firstpositioner PM configured to accurately position the patterning device inaccordance with certain parameters, a substrate table (e.g., a wafertable) WT constructed to hold a substrate (e.g., a resist coated wafer)W and connected to a second positioner PW configured to accuratelyposition the substrate in accordance with certain parameters, and aprojection system (e.g., a refractive projection lens system) PLconfigured to project a pattern imparted to the radiation beam B bypatterning device MA onto a target portion C (e.g., including one ormore dies) of the substrate W.

The illumination system may include various types of optical components,such as refractive, reflective, magnetic, electromagnetic, electrostaticor other types of optical components, or any combination thereof, fordirecting, shaping, or controlling radiation.

The support structure supports (i.e., bears the weight of, thepatterning device). It holds the patterning device in a manner thatdepends on the orientation of the patterning device, the design of thelithographic apparatus, and other conditions, such as for examplewhether or not the patterning device is held in a vacuum environment.The support structure can use mechanical, vacuum, electrostatic or otherclamping techniques to hold the patterning device. The support structuremay be a frame or a table, for example, which may be fixed or movable asrequired. The support structure may ensure that the patterning device isat a desired position, for example with respect to the projectionsystem. Any use of the terms “reticle” or “mask” herein may beconsidered synonymous with the more general term “patterning device.”

The term “patterning device” used herein should be broadly interpretedas referring to any device that can be used to impart a radiation beamwith a pattern in its cross-section such as to create a pattern in atarget portion of the substrate. It should be noted that the patternimparted to the radiation beam may not exactly correspond to the desiredpattern in the target portion of the substrate such as, for example ifthe pattern includes phase-shifting features or so called assistfeatures. Generally, the pattern imparted to the radiation beam willcorrespond to a particular functional layer in a device being created inthe target portion, such as, for example, an integrated circuit.

The patterning device may be transmissive or reflective. Examples ofpatterning devices include masks, programmable mirror arrays, andprogrammable LCD panels. Masks are well known in lithography, andinclude mask types such as binary, alternating phase-shift, andattenuated phase-shift, as well as various hybrid mask types. An exampleof a programmable mirror array employs a matrix arrangement of smallmirrors, each of which can be individually tilted so as to reflect anincoming radiation beam in different directions. The tilted mirrorsimpart a pattern in a radiation beam, which is reflected by the mirrormatrix.

The term “projection system” used herein should be broadly interpretedas encompassing any type of projection system, including refractive,reflective, catadioptric, magnetic, electromagnetic and electrostaticoptical systems, or any combination thereof, as appropriate for theexposure radiation being used, or for other factors such as the use ofan immersion liquid or the use of a vacuum. Any use of the term“projection lens” herein may be considered as synonymous with the moregeneral term “projection system”.

As depicted herein, the apparatus is of a transmissive type (e.g.,employing a transmissive mask). Alternatively, the apparatus may be of areflective type (e.g., employing a programmable mirror array of a typeas referred to above, or employing a reflective mask).

The lithographic apparatus may be of a type having two (dual stage) ormore substrate tables (and/or two or more mask tables). In such“multiple stage” machines the additional tables may be used in parallel,or preparatory steps may be carried out on one or more tables while oneor more other tables are being used for exposure.

The lithographic apparatus may also be of a type where at least aportion of the substrate may be covered by a liquid having a relativelyhigh refractive index, e.g., water, so as to fill a space between theprojection system and the substrate. An immersion liquid may also beapplied to other spaces in the lithographic apparatus such as, forexample, between the mask and the projection system. Immersiontechniques are well known in the art for increasing the numericalaperture of projection systems. The term “immersion” as used herein doesnot mean that a structure, such as a substrate, must be submerged inliquid, but rather only means that liquid is located between theprojection system and the substrate during exposure.

Referring to FIG. 1, the illuminator IL receives a radiation beam from aradiation source SO. The source and the lithographic apparatus may beseparate entities, such as, for example, when the source is an excimerlaser. In such cases, the source is not considered to form part of thelithographic apparatus and the radiation beam is passed from the sourceSO to the illuminator IL with the aid of a beam delivery system BDinclude, for example, suitable directing mirrors and/or a beam expander.In other cases the source may be an integral part of the lithographicapparatus such as, for example when the source is a mercury lamp. Thesource SO and the illuminator IL, together with the beam delivery systemBD (if required) may be referred to as a radiation system.

The illuminator IL may include an adjuster AD for adjusting the angularintensity distribution of the radiation beam. Generally, at least theouter and/or inner radial extent (commonly referred to as σ-outer andσ-inner, respectively) of the intensity distribution in a pupil plane ofthe illuminator can be adjusted. In addition, the illuminator IL mayinclude various other components, such as, for example, an integrator INand a condenser CO. The illuminator may be used to condition theradiation beam in order to have a desired uniformity and intensitydistribution in its cross section.

The radiation beam B is incident on the patterning device (e.g., maskMA), which is held on the support structure (e.g., mask table MT), andis patterned by the patterning device. Having traversed the mask MA, theradiation beam B passes through the projection system PL, which focusesthe beam onto a target portion C of the substrate W. With the aid of thesecond positioner PW and position sensor IF (e.g., an interferometricdevice, linear encoder, 2-D encoder or capacitive sensor), the substratetable WT can be moved accurately (e.g., so as to position differenttarget portions C in the path of the radiation beam B). Similarly, thefirst positioner PM and another position sensor (which is not explicitlydepicted in FIG. 1) can be used to accurately position the mask MA withrespect to the path of the radiation beam B (e.g., after mechanicalretrieval from a mask library, or during a scan). In general, movementof the mask table MT may be realized with the aid of a long-strokemodule (coarse positioning) and a short-stroke module (finepositioning), which form part of the first positioner PM. Similarly,movement of the substrate table WT may be realized using a long-strokemodule and a short-stroke module, which form part of the secondpositioner PW. In the case of a stepper (as opposed to a scanner) themask table MT may be connected to a short-stroke actuato or may befixed. Mask MA and substrate W may be aligned using mask alignment marksM1, M2 and substrate alignment marks P1, P2. Although the substratealignment marks as illustrated occupy dedicated target portions, theymay be located in spaces between target portions (these are known asscribe-lane alignment marks). Similarly, in situations in which morethan one die is provided on the mask MA, the mask alignment marks may belocated between the dies.

The depicted apparatus can be used in at least one of the followingmodes:

1. In a step mode, the mask table MT and the substrate table WT are keptessentially stationary, while an entire pattern imparted to theradiation beam is projected onto a target portion C at one time (i.e., asingle static exposure). The substrate table WT is then shifted in the Xand/or Y direction so that a different target portion C can be exposed.In the step mode, the maximum size of the exposure field limits the sizeof the target portion C imaged in a single static exposure.

2. In a scan mode, the mask table MT and the substrate table WT arescanned synchronously while a pattern imparted to the radiation beam isprojected onto a target portion C (i.e., a single dynamic exposure). Thevelocity and direction of the substrate table WT relative to the masktable MT may be determined by the (de-)magnification and image reversalcharacteristics of the projection system PL. In the scan mode, themaximum size of the exposure field limits the width (in the non-scanningdirection) of the target portion in a single dynamic exposure, whereasthe length of the scanning motion determines the height (in the scanningdirection) of the target portion.

3. In another mode, the mask table MT is kept essentially stationaryholding a programmable patterning device, and the substrate table WT ismoved or scanned while a pattern imparted to the radiation beam isprojected onto a target portion C. In this other mode, generally apulsed radiation source is employed and the programmable patterningdevice is updated as required after each movement of the substrate tableWT or in between successive radiation pulses during a scan. This mode ofoperation can be readily applied to maskless lithography that utilizesprogrammable patterning device, such as, for example, a programmablemirror array of a type as referred to above.

Combinations and/or variations on the above described modes of use orentirely different modes of use may also be employed.

As shown in FIG. 2, the lithographic apparatus LA, which may be usedwith embodiments of the present invention, forms part of a lithographiccell LC, also sometimes referred to a lithocell or cluster, which alsoincludes an apparatus to perform pre- and post-exposure processes on asubstrate. Conventionally these include spin coaters SC to depositresist layers, developers DE to develop exposed resist, chill plates CHand bake plates BK. A substrate handler, or robot RO, picks upsubstrates from input/output ports I/O1, I/O2, moves them between thedifferent process apparatus and delivers them to the loading bay LB ofthe lithographic apparatus. These devices, which are often collectivelyreferred to as the track, are under the control of a track control unitTCU which is itself controlled by the supervisory control system SCS,which also controls the lithographic apparatus via lithography controlunit LACU. Thus, the different apparatus can be operated to maximizethroughput and processing efficiency.

In order to ensure that the substrates that are exposed by thelithographic apparatus are exposed correctly and consistently, it isdesirable to inspect exposed substrates to measure properties such asoverlay errors between subsequent layers, line thicknesses, criticaldimensions (CD), etc. If errors are detected, adjustments may be made toexposures of subsequent substrates, especially if the inspection can bedone soon and fast enough that other substrates of the same batch arestill to be exposed. Also, already exposed substrates may be strippedand reworked—to improve yield—or discarded, thereby avoiding performingexposures on substrates that are known to be faulty. In a case whereonly some target portions of a substrate are faulty, further exposurescan be performed only on those target portions which are consideredgood.

An inspection apparatus is used to determine the properties of thesubstrates, and in particular, how the properties of differentsubstrates or different layers of the same substrate vary from layer tolayer. The inspection apparatus may be integrated into the lithographicapparatus LA or the lithocell LC or may be a stand-alone device. Toenable rapid measurements, it is desirable that the inspection apparatusmeasure properties in the exposed resist layer immediately after theexposure. However, the latent image in the resist has a very lowcontrast (there is only a very small difference in refractive indexbetween the parts of the resist which have been exposed to radiation andthose which have not) and not all inspection apparatus have sufficientsensitivity to make useful measurements of the latent image. Therefore,measurements may be taken after the post-exposure bake step (PEB) whichis customarily the first step carried out on exposed substrates andincreases the contrast between exposed and unexposed parts of theresist. At this stage, the image in the resist may be referred to as“semi-latent.” It is also possible to make measurements of the developedresist image (at which point either the exposed or unexposed parts ofthe resist have been removed) or after a pattern transfer step such asetching. The latter possibility limits the possibilities for rework offaulty substrates but may still provide useful information.

FIG. 3 depicts an example scatterometer, which may be used inembodiments of the present invention. The scatterometer includes abroadband (white light) radiation projector 2 which projects radiationonto a substrate W. The reflected radiation is passed to a spectrometerdetector 4, which measures a spectrum 10 (intensity as a function ofwavelength) of the specular reflected radiation. From this data, thestructure or profile giving rise to the detected spectrum may bereconstructed by processing unit PU (e.g., by Rigorous Coupled WaveAnalysis and non-linear regression or by comparison with a library ofsimulated spectra as shown at the bottom of FIG. 3). In general, for thereconstruction the general form of the structure is known and someparameters are assumed from knowledge of the process by which thestructure was made, leaving only a few parameters of the structure to bedetermined from the scatterometry data. Such a scatterometer may beconfigured as a normal-incidence scatterometer or an oblique-incidencescatterometer.

Another scatterometer that may be used with embodiments of the presentinvention is shown in FIG. 4. In this device, the radiation emitted byradiation source 2 is collimated using lens system 12 and transmittedthrough interference filter 13 and polarizer 17, reflected by partiallyreflected surface 16 and is focused onto substrate W via a microscopeobjective lens 15, which has a high numerical aperture (NA), preferablyat least 0.9 and more preferably at least 0.95. Immersion scatterometersmay even have lenses with numerical apertures over 1. The reflectedradiation then transmits through partially reflecting surface 16 into adetector 18 in order to have the scatter spectrum detected. The detectormay be located in the back-projected pupil plane 11, which is at thefocal length of the lens system 15; however, the pupil plane may insteadbe re-imaged with auxiliary optics (not shown) onto the detector. Thepupil plane is the plane in which the radial position of radiationdefines the angle of incidence and the angular position defines azimuthangle of the radiation. The detector is preferably a two-dimensionaldetector so that a two-dimensional angular scatter spectrum of asubstrate target 30 can be measured. The detector 18 may be, forexample, an array of CCD or CMOS sensors, and may use an integrationtime of, for example, 40 milliseconds per frame.

A reference beam is often used for example to measure the intensity ofthe incident radiation. To do this, when the radiation beam is incidenton the beam splitter 16 part of it is transmitted through the beamsplitter as a reference beam towards a reference mirror 14. Thereference beam is then projected onto a different part of the samedetector 18 or alternatively onto a different detector (not shown).

A set of interference filters 13 is available to select a wavelength ofinterest in the range of, for example, 405-790 nm or even lower, such as200-300 nm. The interference filter may be tunable rather than include aset of different filters. A grating could be used instead ofinterference filters.

The detector 18 may measure the intensity of scattered light at a singlewavelength (or narrow wavelength range), the intensity separately atmultiple wavelengths or integrated over a wavelength range. Furthermore,the detector may separately measure the intensity of transversemagnetic- and transverse electric-polarized light and/or the phasedifference between the transverse magnetic- and transverseelectric-polarized light.

Using a broadband light source (i.e., one with a wide range of lightfrequencies or wavelengths) is possible, which gives a large etendue,allowing the mixing of multiple wavelengths. The plurality ofwavelengths in the broadband preferably each has a bandwidth of Δλ, anda spacing of at least 2·Δλ, (i.e., twice the bandwidth). Several“sources” of radiation can be different portions of an extendedradiation source which have been split using fiber bundles. In this way,angle-resolved scatter spectra can be measured at multiple wavelengthsin parallel. A 3-D spectrum (wavelength and two different angles) can bemeasured, which contains more information than a 2-D spectrum. Thisallows more information to be measured which increases metrology processrobustness. This is described in more detail in EP1,628,164A, which isincorporated by reference herein in its entirety.

The target 30 on substrate W may be a 1-D grating, which is printed suchthat after development, the bars are formed of solid resist lines. Thetarget 30 may be a 2-D grating, which is printed such that afterdevelopment, the grating is formed of solid resist pillars or vias inthe resist. The bars, pillars or vias may alternatively be etched intothe substrate. This pattern is sensitive to chromatic aberrations in thelithographic projection apparatus, particularly the projection systemPL, and illumination symmetry and the presence of such aberrations willmanifest themselves in a variation in the printed grating. Accordingly,the scatterometry data of the printed gratings is used to reconstructthe gratings. The parameters of the 1-D grating, such as line widths andshapes, or parameters of the 2-D grating, such as pillar or via widthsor lengths or shapes, may be input to the reconstruction process,performed by processing unit PU, from knowledge of the printing stepand/or other scatterometry processes.

A key component of accurate lithography is an increased ability tocontrol lithography scanners and scanning functionality (when referringto “scanners” it should be appreciated that this encompasses all thescan modes and functionality described herein and other scanningfunctionalities). Improvements to the scanner's focus and overlay(layer-to-layer alignment) uniformity have recently been achieved by theBaseliner™ scanner stability module, leading to an optimized processwindow for a given feature size and chip application, enabling thecontinuation the creation of smaller, more advanced chips.

When a lithography system is first installed, it must be calibrated toensure optimal operation. However, over time, system performanceparameters will drift. A small amount of drift can be tolerated, butwith too much drift, the system will not likely meet specification.Consequently manufacturers are required to stop production periodicallyfor re-calibration. Calibrating the system more frequently gives abigger process window, but at the cost of more scheduled downtime.

Among other benefits, the scanner stability module greatly reduces theseproduction stoppages. In an embodiment, the scanner stability moduleautomatically drives the system towards a pre-defined baseline on aregular basis (typically every few days). To do this, it retrievesstandard measurements taken from one or more monitor wafers using ametrology tool. The monitor wafer is exposed using a special reticlecontaining special scatterometry marks. From that day's measurements,the scanner stability module determines how far the system has driftedfrom its baseline. It then calculates wafer-level overlay and focuscorrection sets. The lithography system then converts these correctionsets into specific corrections for each exposure on subsequentproduction wafers.

For volume production, it is desirable to have full flexibility whenassigning layers for exposure to a scanner. The alternative,layer-scanner dedication, would put monthly output capacity at risk,since any small disturbance of the lithocluster would directly appear inthe output of that month. One known approach to overcome this risk is byso called (overlay) grid matching. All scanner grids are intentionallyoffset a little, such that all scanners more or less have the same(average) grid for overlay. This grid is often referred to as ‘holy’ or‘golden’ grid. Each product layer can now be exposed on each scanner ofthe same type. This ‘golden’ grid is exposed and etched onto so called‘reference wafers’. If these ‘golden’ matching wafers are used as thebaseline for overlay stability control instead of random monitoringwafers, overlay grid matching and long-term stability can be achieved ina single automated step.

FIG. 5 depicts an embodiment of the overall lithography and metrologymethod incorporating the scanner stability module 500 (essentially anapplication running on a server, in this example). Shown are three mainprocess control loops. In an embodiment, the first loop provides thelocal scanner control using the scanner stability module 500 and monitorwafers. The monitor wafer 505 is shown being passed from the mainlithography unit 510, having been exposed to set the baseline parametersfor focus and overlay. At a later time, metrology unit 515 reads thesebaseline parameters, which are then interpreted by the scanner stabilitymodule 500 so as to calculate correction routines so as to providescanner feedback 550, which is passed to the main lithography unit 510,and used when performing further exposures.

In an embodiment, the second Advanced Process Control (APC) loop is forlocal scanner control on-product (e.g., determining focus, dose, andoverlay). The exposed product wafer 520 is passed to metrology unit 515,where information relating to, for example, the critical dimensions,sidewall angles and overlay is determined and passed onto the AdvancedProcess Control (APC) module 525. This data is also passed to thescanner stability module 500. Process corrections 540 are made beforethe Manufacturing Execution System (MES) 535 takes over, providingscanner control to the main lithography unit 510, in communication withthe scanner stability module 500.

In an embodiment, the third loop is allows metrology integration intothe second APC loop (e.g., for double patterning). The post etched wafer530 is passed to metrology unit 515 which again passes informationrelating to, for example, the critical dimensions, sidewall angles andoverlay, read from the wafer, to the Advanced Process Control (APC)module. The loop continues the same as with the second loop.

In an embodiment, the scanner stability module product definitionassumes that scanner stability module monitor wafers are exposed in lotoperations using scanner settings that are similar to the settings usedfor exposing customer product wafers. In particular, this means thatstandard reticle align (RA), stage align (SA), wafer align (WA) and lotcorrection (LoCo) settings are used for exposing the monitor wafers.Moreover, currently the lot operations interface offers no possibilityfor using non-standard RA/SA/WA/LoCo settings.

However, in test software for certain lithography systems (e.g.,Twinscan®) non-standard RA/SA/WA/LoCo settings are used for some setuptests in order to achieve desired setup accuracy while using only alimited number of wafers for exposure, according to an embodiment of thepresent invention. For example, high-precision SA and WA routines areused in final XY (FXY) testing to calibrate systematic linear offsets(the so-called blue alignment offsets: BAO's) between the actualin-resist overlay and the overlay expected based on the alignment model(SA/WA/RA). High-precision alignment achieves improved SA/WA/RAreproduction by trading throughput (wafers per hour) for this higherprecision. This enables fewer wafers to be used in FXY testing toachieve the specified BAO calibration accuracy, since the number ofwafers required for achieving a given BAO calibration accuracy isproportional to the square of SA/WA/RA reproduction. This is because,statistically speaking, noise of an average is proportional to 1/sqrt(N)to the noise of one measurement, where N is a number of measurementsused to calculate an average. Therefore, if there is a requirement forthe calibration alignment noise to be, for example, ¼ of the alignmentnoise on a single wafer, then at least 16 measurements need to be madefor the calibration (¼=1/sqrt(16)).

In an embodiment, the default RA/SA/WA/LoCo parameters are set forsimultaneous optimization of both overlay and throughput. However, whenexposing the monitor wafers only overlay is of importance, withthroughput being essentially irrelevant as only a few wafers (typicallyfrom 4 to 12) are exposed per week on each stability module monitorcontrolled scanner.

Relatively high default RA/SA/WA/LoCo reproduction is a significantnoise source for the scanner stability module controller, which attemptsto control the scanner using overlay data from the very limited numberof monitor wafers (typically from 4 to 12 wafers per week per scanner).In an embodiment, the scanner stability module implementation achievescontrol accuracy that is sufficient for matched-machine customer usage,but is insufficient for dedicated chuck usage. For example, 2-3 nmscanner stability module control accuracy is small in comparison withmatched-machine XT4-1950 overlay of 7 nm (5.5 nm for NXT) but is verysignificant compared to the XT4-1950 dedicated-chuck overlay of 3.5 nm(2.5 nm for NXT). XT and NXT are different implementations of theTwinscan® apparatus.

Control accuracy could be in principle improved by increasing the numberof monitor wafers, but, the number of monitor wafers is limited by theavailability of the metrology unit, scanner, wafer processing equipmentand the customer FAB automation system, according to an embodiment ofthe present invention. Therefore, noise reduction on the input of thescanner stability module controller should be reduced, in particular, byreducing the noise that is generated by the scanner while performingSA/WA/RA/LoCo alignment in the process of exposing the monitor wafers.

To do this, an interface is implemented on the scanner for selectingnon-standard SA/WA/RA/LoCo settings via lot operations (thereby givinglot operations access to the functionality that currently can beaccessed only via dedicated test software of the test manager). Withsuch an interface in place, the following method can be performed:

For regular monitor wafer exposures, SA/WA/RA/LoCo settings are selectedwhich improve SA/WA/RA/LoCo reproduction (for reasons already explained,this can be done at the cost of decreased throughput). This may includethe following: specifying the number of additional measurement cyclesthat should be performed during high-precision SA/WA/RA/LoCo alignment,possibly in combination with the wait time between the cycles; and,specifying the weights with which the multiple measurements should beaveraged for computing the high-precision SA/WA/RA/LoCo value. By doingso, a sufficient number of measurements are gathered by repeating one ormore alignment schemes several times.

Specifying the RA mode to be used (single-TIS or dual-TIS), where TIS isa Transmission Image Sensor. In normal scanner operation duringproduction, single-TIS alignment is performed to save the time requiredto move the second TIS under the lens, despite the fact that dual-TISmeasurement is more accurate. During monitor wafer exposures, however,the dual-TIS mode may be used as throughput is not critical, therebyachieving better accuracy.

Specifying TIS scan type for RA. A TIS scan samples volume by measuringthe optical image of the TIS marks (that physically reside on thereticle) using the TIS sensor. The goal of the TIS scan is to locate theTIS sensor position in three dimensions with designated characteristics(e.g., maximal TIS sensor output amplitude). Volume sampling is normallytuned to achieve a well-balanced compromise between TIS alignmentaccuracy and time required for the scan. In this case, a different typeof TIS scan may be used when exposing monitor wafers with the aim ofimproving alignment reproducibility for overlay at the cost ofthroughput.

If required, a one-time calibration (either per scanner for each scannerfamily) may be performed so as to determine possible systematic overlayoffsets (e.g., difference in BAOs) between the default SA/WA/RA/LoCosettings and the settings optimized for improved reproduction.

The use of non-standard SA/WA/RA/LoCo settings can be either restrictedto monitor wafer lots (e.g., triggered when a monitor wafer-specific lotID is detected in combination with monitor wafer-specific recipe) ormade generically available to the customer lots as well (for thecustomers that are willing to trade off throughput for improved overlayperformance).

Although specific reference may be made in this text to the use oflithographic apparatus in the manufacture of ICs, it should beunderstood that the lithographic apparatus described herein may haveother applications, such as the manufacture of integrated opticalsystems, guidance and detection patterns for magnetic domain memories,flat-panel displays, liquid-crystal displays (LCDs), thin film magneticheads, etc. The skilled artisan will appreciate that, in the context ofsuch alternative applications, any use of the terms “wafer” or “die”herein may be considered as synonymous with the more general terms“substrate” or “target portion,” respectively. The substrate referred toherein may be processed, before or after exposure, in for example atrack (a tool that typically applies a layer of resist to a substrateand develops the exposed resist), a metrology tool, and/or an inspectiontool. Where applicable, the disclosure herein may be applied to such andother substrate processing tools. Further, the substrate may beprocessed more than once, for example, in order to create a multi-layerIC, so that the term substrate used herein may also refer to a substratethat already contains multiple processed layers.

The terms “radiation” and “beam” used herein encompass all types ofelectromagnetic radiation, including ultraviolet (UV) radiation (e.g.,having a wavelength of or about 365, 355, 248, 193, 157 or 126 nm) andextreme ultra-violet (EUV) radiation (e.g., having a wavelength in therange of 5-20 nm), as well as particle beams, such as ion beams orelectron beams.

The term “lens,” where the context allows, may refer to any one orcombination of various types of optical components, includingrefractive, reflective, magnetic, electromagnetic and electrostaticoptical components.

While specific embodiments of the present invention have been describedabove, it will be appreciated that embodiments of the present inventionmay be practiced otherwise than as described. For example, embodimentsof the present invention, or at least the inventive aspect thereof, maytake the form of a computer program containing one or more sequences ofmachine-readable instructions describing a method as disclosed above, ora data storage medium (e.g., semiconductor memory, magnetic or opticaldisk) having such a computer program stored therein.

The descriptions above are intended to be illustrative, not limiting.Thus, it will be apparent to one skilled in the art that modificationsmay be made to embodiments of the present invention as described withoutdeparting from the scope of the claims set out below.

It is to be appreciated that the Detailed Description section, and notthe Summary and Abstract sections, is intended to be used to interpretthe claims. The Summary and Abstract sections may set forth one or morebut not all exemplary embodiments of the present invention ascontemplated by the inventor(s), and thus, are not intended to limit thepresent invention and the appended claims in any way.

Embodiments of the present invention has been described above with theaid of functional building blocks illustrating the implementation ofspecified functions and relationships thereof. The boundaries of thesefunctional building blocks have been arbitrarily defined herein for theconvenience of the description. Alternate boundaries can be defined solong as the specified functions and relationships thereof areappropriately performed.

The foregoing description of the specific embodiments will so fullyreveal the general nature of the invention that others can, by applyingknowledge within the skill of the art, readily modify and/or adapt forvarious applications such specific embodiments, without undueexperimentation, without departing from the general concept ofembodiments of the present invention. Therefore, such adaptations andmodifications are intended to be within the meaning and range ofequivalents of the disclosed embodiments, based on the teaching andguidance presented herein. It is to be understood that the phraseologyor terminology herein is for the purpose of description and not oflimitation, such that the terminology or phraseology of the presentspecification is to be interpreted by the skilled artisan in light ofthe teachings and guidance.

The breadth and scope of embodiments of the present invention should notbe limited by any of the above-described exemplary embodiments, butshould be defined only in accordance with the following claims and theirequivalents.

What is claimed is:
 1. A lithographic apparatus comprising: anillumination system configured to condition a radiation beam; a supportconfigured to support a patterning device, the patterning device beingcapable of imparting the radiation beam with a pattern in itscross-section to form a patterned radiation beam; a substrate tableconfigured to hold a substrate; a projection system configured toproject the patterned radiation beam onto a target portion of thesubstrate; and a scanning control module configured to control at leastone of the support, the substrate table, and the projection system byretrieving measurements that define control parameters from one or moremonitor wafers to determine a parameter drift from baseline controlparameters, wherein the scanning control module is further configuredto: specify weights applied to measurements from a number of measurementcycles taken during an initial exposure of the one or more monitorwafers, specify one or more wait times between the measurement cycles,and average the weighted measurements to determine the baseline controlparameters.
 2. The apparatus of claim 1, wherein the scanning, controlmodule is configured to apply an alignment model, the alignment modelcomprising SA, WA, RA, and LoCo settings, wherein SA is stage align, WAis wafer align, RA is reticle align and LoCo is lot correction.
 3. Theapparatus of claim 1, wherein the scanning control module is configuredto perform the initial exposure of the one or more monitor wafers usingnon-standard alignment model settings selected for accuracy regardlessof a throughput penalty.
 4. The apparatus of claim 1, wherein thescanning control module is configured to access a non-standard alignmentmodel functionality.
 5. The apparatus of claim 4, wherein thenon-standard alignment model functionality is used by a test module forthe lithographic apparatus.
 6. The apparatus of claim 5, wherein thenon-standard alignment model functionality is available during normalexposure operation to allow throughput of the apparatus to be sacrificedfor accuracy during the normal exposure operation.
 7. The apparatus ofclaim 1, wherein the scanning control module is configured to specify areticle align routine.
 8. The apparatus of claim 7, wherein the scanningcontrol module is configured to specify a transmission image sensor scantype when performing the reticle align routine.
 9. The apparatus ofclaim 1, wherein the scanning control module is configured to perform aone-time calibration to determine systematic overlay offsets betweendelimit standard alignment model settings and non-standard alignmentmodel settings optimized for accuracy.
 10. The apparatus of claim 1,wherein the scanning control module comprises an inspection device witha scatterorneter configured to retrieve the baseline measurements. 11.The apparatus of claim 1, wherein the apparatus is configured to operatein a scanning mode when the patterned radiation beam is scanned acrossthe target portion of the substrate.
 12. A method of producing one ormore monitor wafers for a lithographic apparatus, the one or moremonitor wafers being used in combination with a scanning control module,the method comprising: initially exposing at least one monitor waferusing the lithographic apparatus; applying weights to measurements fromeach of a number of measurement cycles taken during the initial exposureof the at least one monitor wafer; specifying one or more wait timesbetween the measurement cycles; averaging the weighted measurements todetermine initial baseline control parameters; and periodicallyretrieving measurements from the at least one monitor wafer that defineupdated baseline control parameters to determine parameter drift fromthe initial baseline control parameters.
 13. The method of claim 12,wherein the initially exposing comprises applying an alignment model,the alignment model comprising SA, WA, RA, and LoCo settings, wherein SAis stage align, WA is wafer align, RA is reticle align and LoCo is lotcorrection.
 14. The method of claim 12, wherein the initially exposingcomprises using a non-standard alignment model functionality selectedfor accuracy regardless of the throughput penalty.
 15. The method ofclaim 14, wherein the non-standard alignment model functionalitycomprises functionality normally used in performing test routines on thelithographic apparatus.
 16. The method of claim 14, wherein thenon-standard alignment model functionality is to perform test routinesor to expose monitor wafers.
 17. The method of claim 14, wherein thenon-standard alignment model functionality is available during normalexposure operation to allow throughput of the apparatus to be sacrificedfor accuracy during the normal exposure operation.
 18. The method ofclaim 12, wherein the initially exposing comprises specifying a reticlealign routine.
 19. The method of claim 18, wherein the initiallyexposing comprises specifying a transmission image sensor scan type whenperforming the reticle align routine.
 20. The method of claim 12,wherein the initially exposing comprises performing a one-timecalibration to determine systematic overlay offsets between defaultstandard alignment model settings and non-standard alignment modelsettings optimized for accuracy.
 21. The method of claim 12, wherein theinitially exposing comprises periodically retrieving baselinemeasurements using an inspection device with a scatterometer.